1. Field of the Invention
The present invention relates to an illumination structure of a transmission liquid crystal display. More particularly, the present invention relates to an illumination structure comprising an assembly of a directional backlight placed on the back side of a transmission liquid crystal panel and a light diffusing member installed on the front side thereof. To further put it in even more detail, the present invention relates to a structure for preventing an external light arriving at the light diffusing member from being reflected thereby.
2. Description of Related Art
The liquid crystal display is characterized in that the structure thereof is flat and the amount of consumed power is small. Therefore, the liquid crystal display is becoming more and more popular and put to practical use mainly in applications such as hand-held calculators and watches in addition to vehicle onboard panels, measurement display units, office automation equipment and television receivers, to mention a few. Since the liquid crystal display has a property of generating no light, however, the use of the liquid crystal display at a dark place is not convenient. In order to make the liquid crystal display easier to see and to make it possible to use the liquid crystal display at a dark place, a backlight for generating illumination lights uniformly is developed to be placed on the back side thereof. An example of such a backlight is disclosed by Tsuruhara in Nikkei Materials (1987) 21. The degree to which the liquid crystal display is easy to see varies, depending upon the brightness of surrounding light (or external light). In particular, in the case of a liquid crystal display of the reflection type, the degree to which the liquid crystal display apparatus is easy to see is greatly affected by external light. In the case of a liquid crystal display of the transmission type adopting a backlight technique, on the other hand, its visual display power does not decrease, even if the level of the external light is low, by virtue of the back illumination by the backlight. In particular, in the case of an outdoor liquid crystal display such as a car onboard panel for which the brightness of the external light changes from the brightness quantity of the sunlight of the day time to that of a night light, the transmission type having,a backlight is by all means required to sustain the high degree to which the liquid crystal display is easy to see.
As described earlier, the liquid crystal display offers a big advantage that the amount of power consumed thereby is small. However, the backlight for back illumination consumes a large amount of power, destroying the merit. Therefore, it is desirable to use a backlight having a high illumination efficiency. For this reason, in some cases, a backlight having an excellent directivity, that is, having a radiated-light distribution close to that of parallel beams is employed. By the way, the liquid crystal display has a variety of operation modes having something to do with the arrangement of liquid crystal molecules. In the case of a twist nematic (TN) mode, a super twist nematic (STN) mode or a birefringence (ECB) moder either of which is widely used, the transmissivity is dependent upon the visual field angle. FIG. 7 is a diagram showing the dependence of the transmissivity upon the visual field angle in the case of the TN mode. The figure shows a graph obtained from measurement of the transmissivity for a slanting direction forming an angle of 45 degrees with a line normal to the display screen. The figure also shows changes in transmissivity for all directions in the entire 360-degree range with the normal line taken as a center. It is obvious from the figure that, in three directions, the transmissivity is good but, in the remaining direction, the transmissivity becomes extremely poor, indicating the existence of the high dependence of the transmissivity upon the visual field angle. When the liquid crystal display is illuminated by a backlight with a good directivity, the incident illumination light goes all but straight ahead on. Accordingly, when observing the display screen from a position on the front side, the amount of light radiated in slanting directions is relatively small. As a result, in the case of a liquid crystal display which has a transmissivity depending upon the visual field angle, the brightness obtained in an observation at a position in a slanting direction becomes poor, giving rise to a problem that the visual display power deteriorates substantially.
In order to solve this problem, a structure wherein a light diffusing layer is placed on the radiating surface of the liquid crystal panel is proposed. An example of such a structure is disclosed in Japanese Patent Laid-Open No. Hei 6-95099. By providing a light diffusing layer, an incident illumination light arriving all but perpendicularly from a source at the back side passes through the liquid crystal panel, being diffused as well as emitted by the light diffusing layer to result in an optical distribution having also sufficient amounts of light in slanting directions. As a result, display brightness can be obtained at a practical level in each slanting direction, allowing the visual display power to be improved even for a liquid crystal panel with the transmissivity thereof dependent upon the visual field angle.
While the conventional light diffusing layer diffuses an incident illumination light, however, it also reflects an external light to a certain degree, giving rise to a problem that the display contrast becomes poor.